Memory device for transferring streams of data

ABSTRACT

A memory circuit ( 14 ) having features specifically adapted to permit the memory circuit ( 14 ) to serve as a video frame memory is disclosed. The memory circuit ( 14 ) contains a dynamic random access memory array ( 24 ) with buffers ( 18, 20 ) on input and output data ports ( 22 ) thereof to permit asynchronous read, write and refresh accesses to the memory array ( 24 ). The memory circuit ( 14 ) is accessed both serially and randomly. An address generator ( 28 ) contains an address buffer register ( 36 ) which stores a random access address and an address sequencer ( 40 ) which provides a stream of addresses to the memory array ( 24 ). An initial address for the stream of addresses is the random access address stored in the address buffer register ( 36 ).

This application is a divisional of prior application Ser. No.10/190,017, filed Jul. 5, 2002, pending; which was a divisional ofapplication Ser. No. 09/745,892, filed Dec. 21, 2000, now U.S. Pat. No.6,418,078, issued Jul. 9, 2002; which was a divisional of applicationSer. No. 08/488,231, filed Jun. 7, 1995, now U.S. Pat. No. 6,188,635,which was a divisional of application Ser. No. 08/362,289 filed Dec. 22,1994, now U.S. Pat. No. 5,636,176, which was a divisional of applicationSer. No. 08/175,478 filed Dec. 29, 1993, now U.S. Pat. No. 5,400,288,which was a continuation of application Ser. No. 07/843,780 filed Feb.28, 1992, abandoned, which was a divisional of application Ser. No.07/512,611 filed Apr. 20, 1990, now U.S. Pat. No. 5,093,807, which was acontinuation of application Ser. No. 07/137,305 filed Dec. 23, 1987,abandoned.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to digital memory circuits.Specifically, the present invention relates to digital memory circuitswhich have particular advantages when used in connection with videoapplications.

BACKGROUND OF THE INVENTION

Digital TV, VCR, and related video applications often utilize a frame orfield memory that stores pixels which together represent an entire frameof video. Such a frame memory is used in producing a variety of specialeffects, such as frame freezing, zoom, pan, split screen monitoring, andthe like. Although a frame memory may be constructed using conventionaldiscrete integrated circuits, such a frame memory is relativelyexpensive, dissipates an undesirably large amount of power, and occupiesan undesirably large amount of space. When such a frame memory istargeted for use in a commercial product, these problems are major ones.Accordingly, a single integrated circuit, either alone or in combinationwith as few other integrated circuits as possible, improves upon a framememory which has been constructed from conventional discrete integratedcircuits.

Prior art integrated circuit devices have attempted to address the framememory problem. However, such devices fail to provide an architecturewhich adequately addresses video application needs. For example, deviceswhich include only a few of the typically needed frame memory functionsmay be used in providing a wide variety of special effects. However,they must be combined with such a large quantity of conventionaldiscrete integrated circuits that little improvement results overconstructing a frame memory entirely from conventional discreteintegrated circuits. On the other hand, a conventional frame memoryintegrated circuit may include a random access memory with completeon-chip address calculation. A video application which utilizes such aframe memory accesses the entire frame memory serially. Thus, framefreeze and split screen monitoring special effects are supported.However, zoom and pan functions are either impossible or impracticalusing such a device.

Accordingly, the industry feels a need for a frame memory integratedcircuit which optimizes circuit architecture to accommodate a widevariety of special effects without requiring a large quantity ofsurrounding integrated circuits.

SUMMARY OF THE INVENTION

Accordingly, it is an advantage of the present invention that a framememory circuit is provided which permits limited random access.Consequently, a device constructed according to the teachings of thepresent invention may be efficiently used to perform a wide variety ofspecial effect video applications.

Another advantage of the present invention is that a memory circuit isprovided which includes a variety of address calculation modes. Thus, aportion of the address calculations for certain special effect functionsmay be transferred to the memory circuit, and a video application whichutilizes such a memory circuit need not allocate processing power tosuch calculations.

The above advantages of the present invention are carried out in oneform by a memory circuit which stores and provides steams of data. Thismemory circuit supports both serial access and random access. A datainput of a random access memory array couples to a data buffer so thatthe data buffer may synchronize operation of the memory array with thestreams of data. An address input of the random access memory arraycouples to an address sequencer which generates a sequence of memoryaddresses that are successively applied to the memory array. An addressbuffer request also couples to the address sequencer. The address bufferregister supplies a random access address to the address sequencer toinitialize the sequence of memory addresses supplied by the addresssequencer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the accompanying drawings, in which like referencenumbers indicate like features throughout the drawings, and wherein:

FIG. 1 illustrates a frame of a video display screen with which thepresent invention may be used;

FIG. 2 shows a block diagram of a memory circuit built according to theteachings of the present invention;

FIG. 3 shows a block diagram of a first alternate embodiment of anaddress generator portion of a memory circuit built according to theteachings of the present invention;

FIG. 4 shows a block diagram of a second alternate embodiment of anaddress generator portion of a memory circuit build according to theteachings of the present invention and connected to a microprocessor toform a system; and

FIG. 5 shows a block diagram of an address sequencer utilized by theaddress generator portion of a memory circuit built according to theteachings of the present invention;

DETAILED DESCRIPTION

FIG. 1 illustrates a video frame 10, such as may appear on a TV tube orother video display terminal. Although frame 10 may appear as acontinuous analog video picture to a viewer, frame 10 may beelectrically represented as a multiplicity of digitized pixels 12. Eachone of the pixels defines parameters, such as color and relativeintensity, for one of a multiplicity of very small dot areas within thepicture of frame 10. Accordingly, the video frame 10 may contain arelatively large number of the digitized pixels 12. For example, a framecontaining 488 columns of pixels 12 by 488 rows of pixels 12 has a totalof 238,144 pixels per frame.

Pixels 12 are typically transmitted or otherwise processed in apredetermined sequential order to preserve the spatial relationshipbetween the pixels 12. For example, in a conventional raster scanapplication, pixels 12 may be transmitted to a memory device for storageor from storage in a memory device to a video display in successiveorder beginning with a pixel 12 a, that represents the pixel 12 in thefirst column of the first row of frame 10, and continuing in successiveorder to a pixel 12 b, which represents the pixel 12 in the last columnof the first row of frame 10. Immediately following the transmission ofpixel 12 b and sync information (not shown), a pixel 12 c, whichrepresents the pixel 12 in the first column of the second row, may betransmitted followed in successive order by the remaining pixels 12contained in the second row of frame 10. Transmission of pixels 12continues in this fashion until a pixel 12 d, which represents the pixel12 in the last column of the last row of frame 10, has been transmitted.Thus, any processing device which knows the timing relationship betweenan arbitrarily located pixel 12 and the beginning pixel 12 a also knowsor can easily calculate the spatial location of such arbitrarily locatedpixel 12 within frame 10.

A digital TV, VCR, or the like may contain a large frame or field memorywhich is capable of storing all of the pixels 12 within frame 10. Pixels12 collectively appear as a serial data stream when they are applied tothe frame memory. Except for special effects, the relative order ofpixels 12 in this serial data stream must generally be preserved whenthey are read from the frame memory to preserve the spatialrelationships between the pixels 12. Some special effects do not requirethis preserved order, and valuable computation time may be wasted byprecisely preserving the order of the pixels 12 as the pixels 12 arebeing read from the frame memory.

One such special effect is a zoom effect wherein a small portion of astored, digitized frame is expanded and converted to an analog signal tofill an entire video display. For example, if frame 10 in FIG. 1represents an entire video display, then an area 11 within frame 10bounded by rows i and j and columns m and n is expanded in a zoomspecial effect to fill the entire frame 10. Thus, in the zoom specialeffect all of the digitized pixels 12 residing within frames 10 outsideof the area 11 bounded by rows i and j and columns m and n are inactiveand may be discarded. In other words, these inactive ones of the pixels12 need not be read from the frame memory. Consequently, the pixel 12located at column m and row i will be utilized as the first active pixel12 a transmitted to the video display in the zoom special effect. Activepixels 12 may be duplicated to complete an entire row of frame 10, androws may be duplicated to complete the vertical component of the zoomeffect. All of the digitized pixels transmitted to the video screen areconverted to an analog signal for display on the video screen.

In a split screen special effect, an entire frame 10 may be shrunk intoa small area 13 of a screen, such as that bounded by row j and the lastrow of frame 10, and column n and the last column of frame 10. Thisspecial effect is accomplished by utilizing only active ones of thepixels 12 out of each of a predetermined number of the pixels 12 from anentire frame 10 of the pixels 12, and ignoring the intervening inactiveones of the pixels 12 (ie. skipping inactive pixels). For the exampledepicted in FIG. 1, the shrunken frame is formed using only the activepixels 12 that reside in one of every three columns and one of everythree rows of the frame 10.

The present invention provides a memory circuit which series as a framememory and permits these and other special effects to be performedefficiently. FIG. 2 shows a block diagram of a memory circuit 14 builtaccording to the teachings of the present invention. In general, thepreferred embodiment of memory circuit 14 represents a single chipintegrated circuit that contains 2.sup.20 or 1,048,576 bits of memorystorage organized as 262,144 four bit wide words with special write andread access arrangements. Accordingly, a sufficient quantity of wordstorage is provided to buffer or store an entire 488.times.488 frame ofthe pixels 12 (see FIG. 1). If more than four bits of precision arerequired to accurately describe each pixel, then additional ones ofmemory circuit 14 may be used to store such additional bits

Memory circuit 14 generally operates in a serial access mode for bothwrite and read operations but has particular features which permitrandom access for writing or reading of the memory circuit 14 on alimited scale. Those skilled in the art will understand that serialaccess refers to a mode of storing and reading data in which the datamust be read out from a memory in the same order sequential address inwhich it was stored into the memory. Furthermore, random access refersto the ability to write, read, or otherwise access any location in amemory array by supplying a selected unique address which corresponds tosuch memory location.

Specifically, for receiving analog video signals converted to digitalpixels, memory circuit 14 includes a serial pixel data input 16 a, whichin the preferred embodiment supplies four bits of data per pixel. Serialpixel data input 16 a couples to an input port of a write serial latch18 a, and an output port of write serial latch 18 a couples to an inputport of a write register 20 a.

An output port of write register 20 a couples to a data input port 22 aof a memory array 24. In the preferred embodiment, memory array 24 is adynamic random access memory (DRAM) array containing 2.sup.18 or 262,144four bit memory locations. A data output port 22 b of memory array 24couples to a data input port of a read register 20 b, and a data outputport of read register 20 b couples to a data input port of a read seriallatch 18 b. A data output port of read serial latch 18 b couples to aserial pixel data output 16 b, which in the preferred embodimentprovides four bits of data per pixel for conversion to an analog videosignal for display.

A serial write clock terminal 26 a couples to a write address generator28 a, an arbitration and control circuit 30, and a clock input or writeserial latch 18 a. Similarly, a serial read clock terminal 26 b couplesto a read address generator 28 b, arbitration and control circuit 30,and a clock input of read serial latch 18 b. A refresh address andtiming circuit 32 has an output which couples to an input of arbitrationand control circuit 30, and outputs 21 a, 21 b, 23, and 25 fromarbitration and control circuit 30 respectively couple to a clock inputof write register 20 a, a clock input of read register 20 b, a controlinput of memory array 24, and an address input of memory array 24.Serial write clock terminal 26 a and serial read clock terminal 26 breceive respective write and read continuous clock signals each formedof rising and falling edges regularly spaced in time. The write and readclock signals are continuous during operation of memory circuit 14.

As shown in FIG. 2, address generators 28 a and 28 b comprisesrespective write and read address ports that are structurally similar toone another in the preferred embodiment. Thus, a write control dataterminal 34 a couples to a serial data input of an address bufferregister 36 a in write address generator 28 a. A read control dataterminal 34 b couples to a serial data input of an address bufferregister 36 b in read address generator 23 b. Likewise, a write controlstrobe terminal 38 a couples to a clock input of address buffer register36 a, and a read control strobe terminal 38 b couples to a clock inputof address buffer register 36 b. A data output of address bufferregister 36 a couples to a data input of an address sequencer 40 a, anda data output of address buffer register 36 b couples to a data input ofan address sequencer 40 b. A write reset terminal 42 a couples to aclear input of address sequencer 40 a, and a write transfer terminal 44a couples to a present input of address sequencer 40 a. A read resetterminal 42 b couples to a clear input of address sequencer 40 b, and aread transfer terminal 44 b couples to a preset input of addresssequencer 40 b. Serial write clock terminal 26 a couples to a clockinput of address sequencer 40 a within address generator 28 a, andserial read clock terminal 26 b couples to a clock input of addresssequencer 40 b within address generator 28 b. An output 46 a of addresssequencer 40 a presents the output signal from address generator 28 aand couples to an input of arbitration and control circuit 30. Likewise,an output 46 b of address sequencer 40 b presents the output signal fromaddress generator 20 b and couples to arbitration and control circuit30. Memory circuit 14 may be provided in a 20 pin integrated circuitpackage.

As discussed above, memory circuit 14 may be operated in either a serialor a limited random access mode. In addition, the storing or writing ofdata into memory circuit 14 may occur asynchronously with the reading orproviding of data from memory circuit 14. Asynchronous means timed byother than a common clock. Memory circuit 14 may be written intoserially by activating write reset signal on terminal 42 a to clearaddress sequencer 40 a. Then, a four bit wide stream of serial data maybe stored in memory circuit 14 by applying the four bit data nibbles atthe write clock rate to the data input 16 a while asserting a serialwrite clock signal at terminal 26 a. One assertion of the serial writeclock signal causes write serial latch 18 a to temporarily store orbuffer one four bit data nibble. Write serial latch 18 a operates as afour bit wide shift register. Thus, subsequent four bit nibbles from thedata stream of serial pixel data applied at data input 16 a are shiftedinto serial latch 18 a at the write clock rate upon subsequentassertions of the serial write clock signal.

In addition, each assertion of the serial write clock signal also causesaddress sequencer 40 a of write address generator 28 a to supply a newselected random access address to arbitration and control circuit 30. Inother words, address sequencer 40 a provides a stream of addresses toarbitration and control circuit 30 which corresponds to the stream ofdata being stored in write serial latch 18 a.

Arbitration and control circuit 30 receives addresses from addressgenerators 28 a-28 b and refresh address and timing circuit 32

Circuit 30 monitors these inputs and various timing signals to decidewhich of the addresses provided on these inputs should be transferred ata specific time to memory array 24. Arbitration and control circuit 30includes conventional logic circuits for controlling the timingoperation of the dynamic memories which comprise memory array 24. Thus,arbitration and control circuit 30 passes an address generated byaddress generator 28 a to memory array 24 so that data may be writteninto memory array 24, but a delay may occur due to refresh operations orread accesses of memory array 24. Accordingly, arbitration and controlcircuit 30 may additionally contain storage devices so that addressesgenerated by address generators 28 a-28 b are not lost when immediateaccess to memory array 24 is blocked. When arbitration and controlcircuit 30 identifies a time at which the serial pixel data may bewritten into memory array 24, such data is transferred from write seriallatch 18 a into write register 20 a and then written into memory array24. Accordingly, write serial latch 18 a and write register 20 atogether represent a double buffering scheme which permits asynchronousoperation of memory array 24 and particularly the storing of serialpixel data into memory circuit 14.

The reading of data from memory array 24 occurs in a manner similar tothat described above for the storing of data onto memory array 24. Thus,an address generated by address generator 28 b is transferred througharbitration and control circuit 30 at an appropriate time to cause datafrom memory array 24 to be read into read register 20 b. Thereafter,this data is transferred into read serial latch 18 b so that such datamay be provided at data output terminal 16 b through the application ofa serial read clock signal at terminal 26 b. Serial data is provided atoutput 16 b asynchronously with the operation of memory array 24 andasynchronously with the storing of serial pixel data into memory circuit14 at terminal 16 a.

The limited random access feature of memory circuit 14 is providedthrough address generators 28 a-28 b. In the embodiment of memorycircuit 14 shown in FIG. 2, write address generator 28 a and readaddress generator 28 b are structurally and operationally identical,except that write address generator 28 a provides write addresses whileread address generator 28 b provides read addresses. Accordingly, bothaddress generators 28 a-28 b are described below by reference only towrite addresses generator 28 a. Those skilled in the art will recognizethat read address generator 28 b operates identically in the preferredembodiment.

A random access address may be serially loaded into address bufferregister 36 a by applying such address to control data terminal 34 a ina sequential manner and activating a control strobe signal applied atterminal 38 a when valid data appear at terminal 34 a. Thus, in theembodiment shown in FIG. 2, address buffer register 36 a represents aserial shift register. The use of a serial shift register conserves thenumber of external pins needed for constructing memory circuit 14 in anintegrated circuit when compared to a parallel loaded register. Afterthe random access address has been entered into address buffer register36 a, it may be transferred to address sequencer 40 a by the applicationof a write transfer signal at terminal 44 a. In the preferredembodiments of the present invention, address sequencer 40 a mayrepresent a presetable, binary counter or other presetable sequencingcircuit. Thus, the transferred address forms the initial address of asequence of addresses which are subsequently generated by addressgenerator 28 a. If address sequencer 40 a represents a binary counter,then subsequent addresses will increment or decrement starting with thispresent or initial value.

If memory array 24 contains 2.sup.18 four bit words of memory, thenaddress buffer register 36 a may advantageously represent an 18 bitregister, and address sequencer 40 a may represent an 18 bit counter, orother sequencing circuit. On the other hand, address buffer register 36a and address sequencer 40 a may contain fewer bits, such as nine bitsfor example. In the nine bit situation, the random access addressprovided by address buffer register 36 a could access the beginning ofmemory pages or rows wherein each page or row contains 2.sup.9 or 512words of memory.

The inclusion of address buffer register 36 a to provide a limitedrandom access feature permits memory circuit 14 to be efficientlyutilized in a zoom special effect. For example, a zoom effect may beaccomplished by writing an entire frame of pixel data into memory array24 using a serial write access mode. A beginning, preset or initialpixel address, such as the address of a pixel located at row i column m,in FIG. 1, may then be loaded into read address buffer register 36 b andtransferred to address sequencer 40 b. A first row, such as row i, ofthe portion of frame 10 which is to be expanded into an entire frame maythen be read from memory array 24 in a serial or sequential mode until apixel corresponding to, for example, row i, column n, appears at outputterminal 16 b. Readout occurs at the serial read clock rate. A row maybe repeated as often as necessary to achieve vertical zoom bytransferring the random access address from address buffer register 36 bto address sequencer 40 b. An address corresponding to the pixel locatedat row i+1 and column m may then be loaded into address buffer register36 b and transferred to address sequencer 40 b. This process continuesat the serial read clock rate until a final pixel for the frame to beexpanded has been output from memory array 24. The pixels are convertedto analog video signals for display. Due to this feature, a video systemneed not start accesses of memory circuit 12 at an initial address, suchas pixel 12 a (shown in FIG. 1) and access inactive pixels stored withinmemory array 24. More efficient operation results.

The present invention contemplates alternate embodiments of addressgenerators 28 a-28 b. A first alternate embodiment of address generators28 a-28 b is shown in FIG. 3. FIG. 3 shows only one of addressgenerators 28. The address generator 28 shown in FIG. 3 may serve aseither write address generator 28 a or read address generator 28 b (seeFIG. 2).

In this first alternate embodiment of an address generator 28, addressbuffer register 36 may be loaded both serially and in parallel. Thus,control data terminal 34, which may represent either write control dataterminal 34 a or read control data terminal 34 b, as discussed above inconnection with FIG. 2, couples to the serial data input of addressbuffer register 36. Control strobe terminal 38 couples to the serialclock input of address buffer register 36 and a serial clock input of anaddress offset register 48. The parallel data output of address bufferregister 36 couples to a first input of an adder 50 and the data inputof address sequencer 40. A parallel data output of address offsetregister 46 couples to a second input of adder 50. An output of adder 50couples to a parallel data input of address buffer register 36, andtransfer terminal 44 couples to a parallel clock input of address buffer36 and the preset input of address sequencer 40. A most significant bitfrom the parallel data output or a serial output bit, of address bufferregister 36 couples to a serial data input of address offset register48. Serial clock terminal 26 couples to the clock input of addresssequencer 40, and reset terminal 42 couples to a clear input of addresssequencer 40. A data output of address sequencer 40 couples to addressgenerator output 46.

Address buffer register 36 and address sequencer 40 operate in thisfirst alternate embodiment similarly to their above-described operationin connection with address generator 28 a-28 b of FIG. 2. However, inthis first alternate embodiment, the control data provided at terminal34 is used to load both address buffer register 36 and address offsetregister 48. Thus, additional bits of control data are loaded intomemory circuit 14 without requiring additional integrated circuit pins.Moreover, a most significant bit, or a serial output bit 51, fromaddress offset register 48 may advantageously be routed to the controldata input for the other one of read and write address generators 28 aand 28 b (see FIG. 1). In addition, the control strobe signal applied atterminal 38 may be routed to the other one of control strobe terminals38 a and 38 b of FIG. 2. These two connections between addressgenerators 28 a and 28 b eliminate two integrated circuit pins from thestructure shown in FIG. 2.

In this first alternate embodiment of the present invention, the controldata contained in address offset register 48 is added to a currentinitial address value contained in address buffer register 36 to providea new initializing random access address value. This new initializingvalue is loaded into address buffer register 36 when the current addressvalue is transferred into address sequencer 40.

Referring additionally to FIG. 1, the first alternate embodiment of thepresent invention may be advantageous in performing, for example, thezoom special effect. Thus, the address offset value loaded into addressoffset register 48 may represent the quantity of inactive pixelsoccurring between column n of one row and column m of the next row. Atthe end of each frame row a transfer signal may be asserted on terminal44, and the random access address of the next active pixel,corresponding to column n of the next row, is automatically calculatedand stored in address buffer register 36 to initiate another sequence ofsequential accesses to memory circuit 14. Complexity of a video systememploying memory circuit 14 decreases because components external tomemory circuit 14 need not calculate this address.

A second alternate embodiment of address generators 28 a-28 b from FIG.2 is shown in FIG. 4. The FIG. 4 embodiment illustrates that randomaccess addresses may be loaded into address buffer register 36 in aparallel fashion, which may be more compatible with conventionalmicroprocessor integrated circuits. However, the number of integratedcircuit pins needed to implement this embodiment increases over theembodiments discussed above in connection with FIGS. 2 and 3. Inaddition, FIG. 4 shows the inclusion of an alternate address bufferregister 52 in addition to address buffer register 36. Specifically,control data terminals 34 may advantageously provide an eight bitmicroprocessor data bus 80 which couples to data inputs of individualeight bit portions 54 a, 54 b, and 54 c of address buffer register 36.In addition, control data terminals 34 couple to data inputs ofindividual eight bit portions 56 a, 56 b, and 56 c of alternate addressbuffer register 52. Data outputs of individual portions 54 a-54 ctogether form a 24 bit bus which couples to a first data input of amultiplexer 58. Likewise, data outputs of individual portions 56 a-56 cform a 24 bit bus which couples to a second data input of multiplexer58. A data output of multiplexer 58 couples to a data input of a binarycounter which serves as address sequencer 40 in this second alternateembodiment. Of course, those skilled in the art will recognize that thenumber of subregisters included within address buffer register 36 andalternate address buffer register 52 and the number of bits containedwithin the buses described above are subject to a substantial variationin accordance with specific application requirements.

In addition, microprocessor address input terminals 60 a, 60 b, and 60c, couple to address input of a decoder 62 and an address input terminal60 d couples to an enable input of decoder 62. The control strobeterminal 38, discussed above, couples to an enable input of decoder 62.Outputs 01-06 of decoder 62 couple to clock inputs of individual addressbuffer register portions 54 a-54 c and clock inputs of individualalternate address buffer register portions 56 a-56 c, respectively. Anoutput 07 from decoder 62 couples to a clock input of a flip flop 64which is configured to toggle upon the activation of the clock input. Anoutput of flip flop 64 couples to a select input of multiplexer 58. Anoutput 08 of decoder 62 couples to a preset input of binary counter 40.The serial clock 26 couples to a clock input of binary counter 40, andreset terminal 42 couples to a clear input of flip flop 64 and a clearinput of binary counter 40. An output of binary counter 40 couples tooutput 46 of address generator 28.

In this second alternate embodiment of address generator 28, oneinitializing random access address may be stored in address register 36while an alternate initializing random access address is stored inalternate address buffer register 52. A microprocessor 82 may storethese addresses in memory circuit 14 through conventional memory or I/Owrite operations to addresses specified by signals applied on terminals60 a-60 c. An address input bit applied at terminal 60 d mayadvantageously distinguish between a write address generator 28 a and aread address generator 28 b (see FIG. 1). By applying an active signalto reset terminal 42, flip flop 64 and binary counter 40 may beinitialized to a cleared state. At this point, address generator 28operates substantially as described above in connection with FIG. 2.However, an alternate random access address stored in alternate addressbuffer 52 may selectively initialize binary counter 40. A microprocessorwrite operation which toggles flip flop 54, followed by a microprocessorwrite operation that transfers data into binary counter 40, initializesbinary counter 40 with an alternate random access address. Flip flop 64may be toggled by performing a write operation to the address whichactivates output 07 of decoder 62. A transfer operation from theselected one of address buffer registers 36 and 52 occurs by writing tothe address which activates the output 08 of decoder 62.

Alternate address buffer register 52 may advantageously be used by avideo system to efficiently buffer a line within a frame of data. Sincememory circuit 14 of the preferred embodiment contains a sufficientquantity of memory to accommodate 2.sup.18 or 262,144 pixels, memorycircuit 14 has unused memory locations when used to store a single frameof data which contains, for example, 480 pixel columns by 480 pixelrows. Accordingly, a random access address in this unused portion ofmemory may be loaded in alternate address buffer register 52. A singleline of a frame may be efficiently stored in memory circuit 14 bytransferring this alternate initial address value to binary counter 40,then sequentially storing such line of pixels into the otherwise unusedportion of memory circuit 14.

In addition, the present invention contemplates alternative embodimentsfor address sequencer 40. As shown in FIG. 4, address sequencer 40 mayrepresent a conventional presetable, clearable, binary counter. Suchcircuits are well known to those skilled in the art and need not bedescribed in detail herein. However, address sequencer 40 mayalternatively represent a circuit which increments or decrements by avariable step value which may differ from the value of one. Such acircuit is shown in FIG. 5.

Accordingly, in FIG. 5 parallel address data input terminals 44 coupleto a first input of an address buffer register 66. Preset terminalcouples to a select input of address buffer register 66. An output 67 ofregister 66 couples to a data input of address sequencer 68, and theclock input terminal 26 of address sequencer 40 couples to a clock inputof sequencer 68. Likewise, the reset or clear terminal 42 couples to aclear input of sequencer 68. A data output of sequencer 68 provides thedata output of address sequencer 40 and additionally couples to a firstinput of an adder 70. An output of adder 70 couples to a second input ofaddress buffer register 66. The address or control data terminals 34,discussed above in connection with FIGS. 2-4, also couple to a datainput of an address increment register 72. Additionally, the controlstrobe terminal 38, discussed above in connection with FIGS. 2-4,couples to a clock input of register 72. A data output of the addressincrement register 72 couples to a second input of adder 70.

In this FIG. 5 embodiment of address sequencer 40, register 72 mayrepresent either a parallel or a serially loaded register, as discussedabove in connection with FIGS. 2-4. Additionally, if register 72represents a serially loaded register, then register 72 may representone register out of many coupled together in a long chain of seriallyloaded registers, as discussed above in connection with FIG. 3. The dataloaded into register 72 is intended to represent a increment step bywhich sequencer 68 generates successive addresses at output 46 ofaddress generator 28. A current output of address sequencer 68 is addedto the step increment value from address increment register 72 in adder70, and routed through buffer register 66 back to sequencer 68. Thus, asubsequent address generated by address sequencer 68 equals the previousaddress plus the address step increment contained in register 72. Thisaddress step increment need not equal the value of integer one but mayequal any positive or negative value. Furthermore, if the number of bitscarried on the buses that couple together register 72, adder 70,register 66, and sequencer 68 is greater than the number of bitsprovided at the output of address sequence. 68, then subsequentaddresses may be incremented in fractional steps.

Address sequencer 68 may be preset, or initialized, with a random accessaddress by applying an active signal on the present terminal 44,supplying data at the data control input terminals 34, and clocking theclock signal of address sequencer 68. Thus, this initializing randomaccess address is loaded directly into sequencer 68. In addition,address sequencer 68 may be cleared, or reset, by applying a resetsignal to the clear input terminal 42.

Referring additionally to FIG. 1, the address sequencer 68 depicted inFIG. 5 is useful in performing the split screen special effect where anentire frame is displayed in only a small portion of a video screen,such as the lower right hand area 13 shown in FIG. 1. With this specialeffect, if memory circuit 14 has every pixel 12 of a frame 10 storedtherein, then only one out of every group of a predetermined number ofstored pixels is active in constructing the shrunken screen. Addresssequencer 68 shown in FIG. 5 allows memory circuit 14 to provide onlythe active pixels by supplying a sequence of addresses which omitsinactived pixel addresses.

In summary, the present invention provides a memory circuit which allowsa video system to efficiently perform special effects. Specifically, theinclusion of various limited random accessing features allows memorycircuit 14 to store and/or provide only active pixels for a givenspecial effect and not inactive pixels. Consequently, active pixels maybe retrieved from memory circuit 14 much quicker than occurs with theuse of prior art frame memory circuits.

The foregoing description uses preferred embodiments to illustrate thepresent invention. However, those skilled in the art will recognize thatchanges and modifications may be made in these embodiments withoutdeparting from the scope of the present invention. For example, readaddress generator 28 b need not precisely resemble write addressgenerator 28 a. Additionally, although the embodiments depicted in FIGS.3-5 are mentioned above as being alternative embodiments, nothingprevents one skilled in the art from combining the teaching from morethan one of these alternate embodiments into a single frame memorycircuit 14. Moreover, those skilled in the art will recognize thatadditional address processing capabilities may be built into framememory circuit 14. Such additional address processing capabilities mayinclude the addition of a signal which indicates the end of a frameline, a signal which indicates the end of a frame, and the automatictransferring of random access addresses to an address sequencer upon theoccurrence of the end of line and end of frame signals. Furthermore,although specific frame and memory array dimensions have been presentedherein to aid in teaching the present invention, it is intended that thepresent invention not be limited to any particular dimensions. These andother modifications obvious to those skilled in the art are intended tobe included within the scope of the present invention.

What is claimed is:
 1. A memory device for transferring streams of datacomprising: A. a random access memory array storing a word of data ateach of plural addressable locations; B. a data port having a number ofdata terminals conducting the streams of data, the data terminals beingcoupled to data buffers so that the streams of data occur asynchronouslywith the operation of the memory array and the data buffers coupling thedata to the memory array; and C. an address port having a number ofaddress terminals less than the number of data terminals, the number ofaddress terminals coupling to an address sequencer receiving addresscontrol data including an initial random access address and the addresssequencer coupling a series of addresses to the memory array startingwith the initial random access address for transferring data to and fromthe memory array, the address port including a control data buffercoupled to the number of address terminals and coupled to the addresssequencer, the control data buffer receiving address control datasignals from the number of address terminals for controlling theaddresses produced from the address sequencer.
 2. The system of claim 1in which the address port includes an alternate address buffer registerthat receives address control data in the form of a new initial randomaccess address while the sequencer couples the series of addresses tothe memory array.
 3. The system of claim 1 in which the control databuffer includes an address offset register that receives address controldata in the form of an address offset step for producing the series ofaddresses to the memory array while the sequencer couples the series ofaddresses to the memory array.
 4. The system of claim 1 in which thecontrol data buffer includes an address increment step register thatreceives address control data in the form of an increment step forproducing the series of addresses to the memory array while thesequencer couples the series of addresses to the memory array.
 5. Thesystem of claim 1 in which there are four data terminals and one addressterminal.